JP 2005-537210A (PTL 1), US 2006/0130530A (PTL 2), JP 2007-504080A, JP2008-536190A, JP 2010-501894A, JP 2009-541796A, JP 2010-526749A, WO 98/002389, and U.S. Pat. No. 5,146,534B describe silica base optical fibers in which cores are doped with alkali metal elements. It is said that in the case where a core portion of an optical fiber perform is doped with an alkali metal element, the viscosity of the core portion can be reduced during drawing of the optical fiber preform, relaxation of a network structure of a silica glass proceeds and, thereby, the attenuation of the optical fiber can be decreased.
PTL 1 and PTL 2 describe a diffusion method as a method for doping the silica glass with the alkali metal element. The diffusion method dopes the inside surface of the silica glass pipe with the alkali metal element through diffusion by heating a pipe with an external heat source or generating plasma in the pipe while a source material vapor obtained by heating an alkali metal or an alkali metal salt, which serves as a source material, is introduced into the silica glass pipe.
After the vicinity of the inside surface of the silica glass pipe is doped with the alkali metal element, as described above, the diameter of the resulting silica glass pipe is reduced by heating. After the reduction in diameter, some thickness of the inside surface of the silica glass pipe is etched for the purpose of removing transition metal elements, e.g., Ni and Fe, which are added at the same time with addition of the alkali metal element. The alkali metal element diffuses faster than the transition metal element. Therefore, even when some thickness of glass surface is etched to remove the transition metal element, it is possible to allow the alkali metal element to remain.
After the etching, the glass pipe is heated and collapsed, so that an alkali metal element-doped rod (core portion of an optical fiber preform) is produced. A glass serving as a cladding portion having a refractive index smaller than the refractive index of the core portion is applied around the perimeter of the core portion, so that the optical fiber preform is produced. Then, an optical fiber can be produced by drawing the resulting optical fiber preform by a known method.